HR: 1340h
AN: V23A-0678    [Abstracts]
TI: Rheology of Suspensions and the Emplacement of Tongues Crystal-Rich Magma Within Sills
AU: Sumita, I
EM: sumita@hakusan.s.kanazawa-u.ac.jp
AF: Department of Earth Sciences, Faculty of Science, Kanazawa University, Kakuma, Kanazawa, 920-1192 Japan
AU: * Manga, M
EM: manga@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley 307 McCone Hall, Berkeley, CA 94720-4767 United States
AB: We measured the rheological properties of suspensions subjected to oscillatory stress. Measurements were made using a cone-and-plate rheometer for spherical plastic particles suspended in Newtonian silicone oil; particle volume fractions between 10 and 60 %, suspending fluid viscosity between 50 mPas and 1.25 Pas, and particle size of 8 and 40 microns. We find that the changes in rheological behavior are best characterized by strain, rather than stress, thresholds. At low strains, the suspension behaves as an elastic solid. For larger strains, greater than ~ 10^-5 to ~ 10^-3 (depending of volume fraction and grain size), the suspension behaves in a fluid-like, as opposed to solid-like, manner. Viscosity decreases with increasing shear and shear rate. For strains greater than about 10^-1 to 10^O, the suspension appears to become dilatant, and is solid-like again. At strains just below the onset of dilatancy, the viscosity is well-described by the Einstein-Roscoe relationship. These changes in behaviour are governed by changes in the microstructure of the particles -- at low and very high strains, the particles form interlocking networks that result in the solid-like behavior. Although the crystallinity of tongues of phenocryst-rich magma within Antarctic dolerite sills is high, viscous flow and deformation within the tongue is still possible and should be expected. We use our rheological measurements to estimate the strain within the tongue. Total strains are predicted to approach about 1, assuming the tongue was extruded all at once. Dilatant features are thus expected, and thin sheets of fine-grained plagioclase (``wispy stringers'') may be one class of structures formed once strains become large enough. Stringers are not distributed symmetrically about the center of the tongue, but are more abundant near its base. We thus infer that magma near the base of the sill experienced more shear strain during ascent and flow within the sill; in contrast, shear strain at the top of the tongue accumulated primarily during flow within the sill (consistent with the stagnation flow cartoon in Figure 4 of Marsh, EOS 2004).
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005